Various proposals have been made heretofore for stopping a machine in response to occurrence of an undesired condition. Thus, for example, U.S. Pat. No. 3,979,930 issued Sept. 14, 1976 in the name of Biester et al. discloses a punch press control system in which a first proximity sensor responds to rotation of the press drive shaft to generate a cyclical timing signal, second, third and fourth proximity sensors generate signals upon, respectively, appearance of a strip of stock at the press input and emergence from the press of, first, a punched product and, second, the strip skeleton, and all such signals are fed to logic circuitry which responds to them to eventually shut down the press when it runs out of strip stock.
The Biester et al. system, has, however, the disadvantage that, upon occurrence of a misfeed of the stock in the sense that the stock does not properly register with the die but nonetheless punched product emerges, the system will not stop the press. As another consideration, while it is not clear from the Biester et al. disclosure how the logic circuitry thereof works, it would appear that, in the event of malfunction of the "timing" proximity sensor so as to fail to produce a signal, the press would nonetheless continue to operate, i.e., would not fail safe.